Electrostatic precipitation is presently generally accepted as the most practical method of separating solid or liquid particulates from moving volumes of gas in commercial processes. Primarily the precipitation process reduces the incidence of effluent air-polluting agents, aiding in the maintenance of acceptable environmental standards while permitting the use of convenient and economical industrial fuels which would otherwise be contraindicated by ecological considerations. Secondarily, some substances recoverable by precipitation may have economic value in themselves.
However, despite the fact that the basics of electrostatic precipitation have been known for some fifty years and notwithstanding the considerable commercial development of the process through the intervening years, the efficiency of precipitators is still below desirable levels. Thus to attain acceptable anti-pollution standards, precipitators presently must be large and expensive. Some theoretical considerations indicate wherein improvement in performance may be achieved.
The efficiency of an electrostatic precipitator is given by the expression EQU eff.=1-e.sup.-(A/Q)v.sbsp.d (i)
where
A=collection area in square meters; PA1 Q=flow rate in cubic meters per second; PA1 v.sub.d =drift velocity in meters per second. PA1 q=charge per particle in coulombs; PA1 E=electric field in volts per meter; PA1 .mu.=dynamic viscosity in kilograms per meter-second; PA1 r=particle raidus in meters. PA1 t=charging time in seconds; PA1 E.sub.o =dielectric constant of vacuum=8.85.times.10.sup.-12 coul/Vm; PA1 q.sub.o =charge of electron =1.6.times.19.sup.-19 coul; PA1 K=ion mobility in square meters per volt-second.
Where A and Q are taken as given constraints, as in an existing installation, one can hope to increase efficiency by increasing the drift velocity v.sub.d. The drift velocity is given by the expression EQU v.sub.d =qE/6.pi..mu.r (ii)
where
Since the viscosity is difficult to adjust, particularly in pre-existing systems, one need increase the average electric field E and particle charge q in order to increase drift velocity. The particles are charged by negative ions formed from ambient gas molecules by means of electron attachment. The charge per particle is given by the expression ##EQU1## for particles of diameter at least 0.5 microns, where E.sub.r =relative dielectric constant of particles;
N=ion concentration in ions per cubic meter;
Equation (iii) indicates that the particle charge is proportional to the field intensity E. Then from equation (ii) the drift velocity varies with the square of the field intensity, and from equation (i) it is seen that the efficiency is a strong function of the field intensity. A considerable increase in efficiency may be obtained by increasing the average electric field within the precipitator duct.
At present, most precipitators operate at average electric field levels of between 5 and 10 kV/cm. This is between 1/6 and 1/3 of the breakdown field of uniform field gaps in air at atmospheric conditions. In conventional precipitators, this large discrepancy between actual field intensity and ideally attainable intensity is inherent and necessary, since the field must perform the function of providing electrons via corona discharge. Such corona charging requires a nonuniform field in the vicinity of the charging electrodes. Because the highest potential levels cannot exceed the breakdown capacity of the system, the requirement of nonuniformity is met only with the unfortunate concomitant of reduced average electric field, hence reduced particle charging ability and reduced ability of the field to transport particles to the collecting electrodes.